1. Field of the Invention
The present invention relates to a technology of touch sensor, and more particularly, to a touch sensor with electrostatic immunity and sensing method thereof.
2. Description of the Related Art
In recent years, due to the development of technology, control buttons, such as buttons of an elevator or a game console, evolve from a mechanical type of button into a touch sensor. FIG. 1 is a circuit diagram depicting a capacitive touch sensor in the prior art. Referring to FIG. 1, the touch sensor includes a sensing electrode 101, a resistor 102 and a sensing-control terminal 103, wherein the sensing electrode 101 in the circuit is equivalent to a grounding capacitor Cx.
FIG. 2 illustrates an operational waveform diagram of node A coupled by the sensing electrode 101 and the resistor 102. Referring to FIG. 1 and FIG. 2, in the beginning, the sensing-control terminal 103 charges the node A to a first preset voltage V20, and then the node A is set to high-impedance. Afterward, since the sensing electrode 101 is equivalent to the grounding capacitor Cx, so that the sensing electrode 101 starts to discharge through the resistor 102. The sensing-control terminal 103 continuously detecting a voltage of node A. When the voltage of node A discharges to a second preset voltage V21, the sensing-control terminal 103 determines whether a finger touches the sensing electrode according to a period when the voltage of node A is discharged from the first preset voltage V20 to the second preset voltage, and then the sensing-control terminal 103 begin to charge the node A.
Referring to FIG. 2, the waveform 201 is a voltage waveform of node A when a finger does not touch the sensing electrode 101, and the waveform 202 is a voltage waveform of node A when a finger touches the sensing electrode 101. According to the waveforms, when a finger touches the sensing electrode 101, the equivalent capacitor of the sensing electrode 101 is increased, so that a discharge time T2 of the waveform 202 is longer than a discharge time T1 of the waveform 201. Therefore, as long as it is determined that the period when the voltage of node A is discharged from the first preset voltage V20 to the second preset voltage V21 is longer than the discharge time T1 by the sensing-control terminal, it can be determined that the sensing electrode 101 is touched.
The surface of the sensing electrode 101 is generally made of plastics, such as a polyethylene, polypropylene, and so on. In this type of the capacitive sensor, the sensing electrode 101 is comparatively easy to be influenced by the electrostatic on its surface material. This kind of plastics, such as a polyethylene, polypropylene, and so on, has a characteristic which the accumulated electrostatic therein is hard to eliminate. FIG. 3 is an operational waveform of the node A when the sensing electrode 101 in the prior art is affected by the electrostatic. Referring to FIG. 3, when a finger operates on the abovementioned plastics, the electrostatic will be gradually induced in or out from the plastics, so that the electrical field of the surface of the sensing electrode 101 will be changed. When the surface material of the sensing electrode 101 has positive charges, the discharge time will be increased, the waveform of the node A should be as waveform 301. When the surface material of the sensing electrode 101 has negative charges, the discharge time will be reduced, and the waveform of the node A should be as waveform 302.
Hence, in above two situations, it should be noted that only detecting the discharge time cannot accurately determine whether an object is close to the sensing electrode, and the apparatus controlled by the conventional touch sensor may be abnormal.